1. Field of the Invention.
The present invention, generally, relates to electrical circuit cards and their manufacture.
Circuit cards today are still called "printed circuit boards" even though they seldom are printed. Regardless of how they are made, they must conform to stringent safety requirements concerning their operating voltages, dielectric breakdown voltages and ability to dissipate heat during operating conditions.
When these safety requirements call for ability to operate under voltages of at least 2500 volts, this dictates minimum dielectric thicknesses, which often present heat dissipation problems. Of course, cooling by forced air or water can be provided, but this arrangement causes other problems.
2. Description of the Prior Art.
U.S. Pat. No. 4,993,148 to Adachi et al. describes a method for forming a circuit board that involves plating conductor layers on a pre-defined dielectric layer that leaves a portion of the conductor exposed for mounting components. This method does not involve applying a copper foil, defining a conductor and via pattern in the copper foil and then using the conductor and via pattern to define the dielectric.
U.S. Pat. No. 4,999,740 to Ilardi et al. describes a circuit board made by building layer of photo-imageable dielectric material on a metal substrate and imaging the dielectric to create wells for mounting components on the metal substrate. It does not involve multiple wiring layers, and it does not provide voltage planes of high current carrying capacity or utilize laminated metal foil as a dielectric removal mask.
U.S. Pat. No. 5,081,562 to Adachi et al. describes a circuit board with a metal plate on which a photo-sensitive insulating layer is formed leaving portions of the metal plate exposed for component mounting areas. The circuit board requires copper plating of conductors and vias. It does not contain soldered interconnections of individual layers.
While the structural arrangements of these prior art circuit boards at first appearance have similarities with the circuit card of the present invention, they differ in material respects. These differences will be understood more clearly from a reading of the following detailed description of the invention, and these differences admit of significant advantages in the manufacturing process and in the use of the completed circuit card.
By way of example, the present invention permits a very cost efficient manufacturing process to be used, namely, the elimination of copper plating of conductors and the elimination of copper plating usually required for all interconnects. This advantage is even more significant in the manufacture of power circuit cards that involve thicker copper and take more manufacturing time.
A further advantage that is significant with the circuit card arrangement of the present invention is in its elimination of the usual requirement of copper plating, which permits higher wiring densities to be obtained. A still further advantage that is available with the present invention over the prior art is in the isolation of the primary and secondary voltage planes that produce improved electrical performance and a reduction in cost.
Yet another advantage of the present invention is obtained by the use of thin coatings for the materials of high dielectric breakdown strength, which thin materials also provide a minimal thermal resistance. When used in conjunction with openings in the dielectric for component mounting, an effective heat dissipating circuit card structure results.
Accordingly, the prior art patents identified above are hereby incorporated by reference.